Squaliformes
Squaliformes is an order of sharks within the subclass Elasmobranchii, commonly known as dogfish or sleeper sharks, distinguished by the absence of an anal fin, two dorsal fins (often bearing spines), five gill slits, the presence of spiracles, and the lack of a nictitating membrane.[1] This order encompasses approximately 130 species across seven families, including Squalidae, Centrophoridae, Etmopteridae, Dalatiidae, Somniosidae, Oxynotidae, and Echinorhinidae (though the latter is sometimes classified separately), with a monophyletic radiation originating in the Lower Cretaceous.[2] These sharks exhibit a wide range of sizes, from the diminutive pygmy sharks of the genus Squaliolus (reaching just 22–27 cm in length) to the massive Greenland shark (Somniosus microcephalus), which can exceed 6 meters and is among the longest-lived vertebrates, potentially surviving over 400 years.[3] Many species are deep-sea inhabitants, occurring in benthic and pelagic zones from coastal shallows to abyssal depths beyond 3,000 meters, with a global distribution spanning tropical to polar waters in all major oceans.[4] Notable adaptations include ovoviviparity in reproduction, dermal denticles providing a rough skin texture, and bioluminescence in several families (such as Etmopteridae and Dalatiidae), where photophores enable counter-illumination for camouflage in the deep sea—a trait that evolved during the Cretaceous period.[2] Phylogenetically, Squaliformes form a well-supported clade within the Squalomorphi, sister to Squatiniformes and Pristiophoriformes, with fossil records dating back to the Late Jurassic—highlighting their ancient lineage among modern sharks.[2][5] Ecologically, they play key roles in marine food webs as mid-level predators, though many face threats from bycatch in fisheries targeting more commercially valuable species, such as the spiny dogfish (Squalus acanthias).[6]Taxonomy
Classification
Squaliformes belongs to the kingdom Animalia, phylum Chordata, class Chondrichthyes, subclass Elasmobranchii, and superorder Squalomorphi.[7] The order was established by Goodrich in 1909 to distinguish these sharks based on shared morphological features.[8] Diagnostic traits of Squaliformes include the presence of two dorsal fins, typically bearing spines, and the absence of an anal fin and nictitating membrane.[9] They possess five gill slits and exhibit a primitive orbitostylic jaw suspension, in which the upper jaw is primarily connected to the cranium via the hyomandibula and anterior ligaments.[9] As of 2025, Squaliformes encompasses approximately 143 valid species across six families, according to Eschmeyer's Catalog of Fishes; the family Echinorhinidae (2 species) is sometimes included as a seventh but is now often classified in the separate order Echinorhiniformes based on molecular evidence.[10][2] Historically, these sharks were initially classified within broader groups such as the Squali of Linnaeus, but Goodrich's 1909 revision separated them into Squaliformes based on distinctive fin structures and jaw morphology.[8] Subsequent refinements by Compagno in 1973 solidified the order's boundaries.[7] Recent molecular phylogenies (e.g., 2015) confirm the monophyly of Squaliformes excluding Echinorhinidae, with insights into family relationships such as the paraphyly of Somniosidae.[2]Families and species
The order Squaliformes includes six core families (plus Echinorhinidae sometimes classified separately), totaling approximately 143 species as of 2025, with ongoing taxonomic revisions potentially adjusting these figures as new research uncovers cryptic diversity and resolves synonyms.[10] These families exhibit varied adaptations to marine environments, particularly deep-sea habitats, contributing to the order's global distribution. Echinorhinidae is noted separately below due to its disputed placement.| Family | Common Name | Number of Species (as of 2025) | Example Species | Distinguishing Features |
|---|---|---|---|---|
| Centrophoridae | Gulper sharks | 15 | Centrophorus squamosus (leafscale gulper shark) | Slender bodies with large mouths for gulping prey; often found on continental slopes.[11] |
| Dalatiidae | Kitefin and pygmy sharks | 10 | Squaliolus laticaudus (spiny pygmy shark) | Tiny size (under 30 cm) to larger forms; some species possess bioluminescent organs for deep-water navigation.[12] |
| Etmopteridae | Lantern sharks | 54 | Etmopterus lucifer (blackbelly lanternshark) | Presence of photophores for bioluminescence, enabling counter-illumination and species-specific patterns in the deep sea; highest species diversity in the order due to adaptations for abyssal life.[13][14] |
| Oxynotidae | Rough sharks | 5 | Oxynotus centrina (angular roughshark) | High-backed bodies covered in coarse, thorn-like denticles; inhabit upper continental slopes.[15] |
| Somniosidae | Sleeper sharks | 17 | Somniosus microcephalus (Greenland shark) | Large-bodied species tolerant of extreme deep-sea conditions, including low temperatures and high pressures; known for slow metabolism and longevity.[16][17] |
| Squalidae | Dogfish sharks | 42 | Squalus acanthias (spiny dogfish) | Slender forms with prominent dorsal spines; some species, like the spiny dogfish, hold commercial value in fisheries.[18] |
| Echinorhinidae (often separate order) | Bramble sharks | 2 | Echinorhinus cookei (prickly shark) | Stout bodies adorned with large, spine-like denticles resembling thorns; rare and poorly known deep-water inhabitants.[19] |